A converter may be roughly classified into two structures of operating in the Nyquist Rate or operating in the Oversampling Rate (OSR), wherein the converter operating in the oversampling rate is benefited by the two frequency response characteristics of performing low pass filtering for signal and performing high pass filtering for noise at the same time, and achieves a goal of separating the signal and the noise and eventually reduce the noise in baseband. With a simple exchange of a filter, the oversampling converter may realize the response characteristic of performing band-pass for the signal and performing band-rejection for the noise in an intermediate frequency, so as to realize the separation of the band-pass signal and the intra-band noise, and meanwhile, finish the filtering of the band-pass signal and conversion of digital and analog signal.
No matter for an oversampling digital-to-analog converter or an oversampling analog-to-digital converter, the interior circuit must have a digital-to-analog converter. In general, the digital-to-analog converter is configured to utilize an selected amount of interior elements to output an analog voltage, and each element may generate the same voltage with the same value to determine an output analog voltage value according to the selected amount of the interior elements, so as to realize a circuit of converting a digital quantity into an analog voltage quantity. However, since differences in element size or characteristic caused by manufacturing crafts, voltage values generated by the interior elements are not actually the same, which causes errors of the conversion from the digital quantity into the analog quantity, and further affects accuracy and performance of the digital-to-analog converter, and affects the oversampling converter. Therefore, the conventional technique has developed a technique which utilizes data weighted averaging to improve declining performance caused by mismatch of the interior selected elements. However, a repetition of selected elements of the conventional data weighted averaging technique causes the mismatch of the elements in spectrum and generates noise voltage, which affects the signal quality and circuit performance. Accordingly, an improvement to the repetition of the selected elements of the conventional technique is to add one or more extra elements to perform the data weighted averaging so as to increase the amount of the elements and reduce the repetition of the selected elements. However, the improvement reduces the noise voltage caused by the mismatch of the elements but the noise voltage does not disappear, and the repetition of the selected elements with a specific input range still occurs, which affects the performance of the digital-to-analog converter.
Therefore, it is necessary to improve the conventional technique.